The efficient functioning of the hip joints is extremely important to the well being and mobility of the human body. Each hip joint is constituted by the upper portion of the upper leg bone (femur) which terminates in an offset bony neck surmounted by a ball-headed portion which rotates within a socket (acetabulum) in the hip bone. Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the cartilage lining the acetabulum so that the ball of the femur and hip bone rub together causing pain and further erosion. Bone erosion causes the bones themselves to attempt to compensate and reshape, thus giving a misshapen joint which may well cease to function altogether.
The replacement of the hip joint by an artificial implant is widely practised, but the implants conventionally used and the conventional methods of implantation can suffer from a number of disadvantages.
Thus, conventional hip implants are usually inserted by resecting the neck of the femur and reaming a comparatively large cavity down the femur to receive a bow-shaped implant surmounted by a ball which is then cemented in place using, for example, an acrylic filler material. The implant is bow-shaped to correspond to the angle which the intact femur ball head makes with the downwardly extending stem of the femur. Examples of such hip implants are shown in GB Patent Specification Nos. 1409053 and 1409054.
It will be appreciated that such an operation requires great skill and expertise on the part of the surgeon who must, for example, use one or more bow shaped rasps of increasing size in an effort to locate the canal down the femur and to create a cavity which, at best, can only approximate to the shape of the implant and which requires cement to secure the implant within the cavity. Furthermore, this can lead to errors in positioning the ball head with respect to the acetabulum or femur, thus preventing the normal range of hip movement and/or causing unwanted bone impaction or uneven leg length.
The bottom portion of the implant, while tapering, is conventionally of non-circular flattened cross-section so as to resist rotational forces within the reamed cavity. Such an implant, if correctly cemented can be comparatively efficient but, if not correctly cemented, or after a long period of use, the cement may work loose, thus allowing movement of the implant and causing bone erosion. Bone erosion can lead to tissue reactions which themselves can lead to further bone destruction. As a result, the joint can be damaged beyond repair.
GB patent Specification No. 1489887 discloses an implant which has a substantially linear stem which is grooved to assist in anchoring the stem in the femur and which stem flares medially at its upper end to give a comparatively thick upper portion terminating in a shoulder and ball head. In use, the neck of the femur is resected and the shoulder rests on the cut edge of the femur. There is thus very little bone retention in the upper part of the femur. The specification discloses the use of drills and a milling cutter for forming the desired femur cavity. The milling cutter makes a medial cut in what little remains of the femur neck by being progressively moved down a guide rod in a pre-drilled bore down the femur stem. The junction between the medial cut and the pre-drilled bore cannot be accurately machined in this manner and the device is only suitable for producing a cut of considerable width and shallow depth, which is all that is necessary when most or all of the bony neck of the femur has been removed.
The practice of removing almost completely the bony neck of the femur, as illustrated in GB No. 1489887, is destructive of bone and against the accepted advantages of conserving as much bone as possible.
A further result of the shape of conventional implants and the removal of much of the bony neck (which in a healthy hip bone provides reinforcement) is the problem of so-called "stress shielding". Thus, with a conventional implant, both compression and torsional loads are being borne by the lower portion of the implant projecting down the stem of the femur and not by the upper bowed portion adjacent the ball. This can lead to fatigue failure of the implant itself and/or undue loading of the adjacent portions of the femur. Because the remaining uppermost portions of the femur are shielded from load they themselves may start to disappear.
These problems with existing implants have led to the development of implants which can be employed without using cement. Such an implant is described in European Patent Application Publication No. 0158534 and comprises a stem portion with a substantially linear axis, at least the lower portion of the stem having a substantially circular cross-section. A wedge shaped portion extends medially from the stem at or near the upper end and is surmounted by a ball head. The wedge shaped portion, preferably of flattened section, thus lodges in use within the bony neck of the femur, most of which can therefore be retained. This ensures maximum bone conservation and assists in overcoming the problems of stress shielding. The linear nature of the stem and the circular cross section ensure that the implant can be readily withdrawn for replacement simply by cutting any bone grafting which has occurred towards the top of the implant in the region of the wedge portion. It is possible to employ such an implant without the use of cement, but this naturally assumes that the cavity for the implant can be accurately prepared, both down the main stem of the femur and in the region of the bony neck.